In recent years, a zirconia (ZrO.sub.2)-based sinter is extensively used as a constituent material of, e.g., ceramic scissors and medical materials taking advantage of its toughness, mold extrusion dies taking advantage of its lubricity, heat-insulating engine parts taking advantage of its heat-insulating properties and thermal expansion characteristics, or oxygen sensors and fuel cells taking advantage of its oxygen ion conductivity. It is known that of such sinters the zirconia-based sinters containing a rare earth metal oxide as a stabilizer have exceedingly high fracture toughness properties as compared with other ceramics. Products taking advantage of this property are being developed enthusiastically. For example, attention is focused on use as a grinding part material, e.g., a grinding medium for use in the mixing or grinding of ceramic materials, metal powders, food-related substances, or the like. Also attracting attention is use as a bracket material for dentition correction which is superior in aesthetic property to bracket materials made of metal, polymer, etc., and combines high strength and toughness.
By the way, the fracture toughness properties of a zirconia-based sinter containing a rare earth metal oxide as a stabilizer tend to decrease in proportion to the amount of the stabilizer and to sintering temperature. For example, it is known that zirconia-based sinters having a stabilizer content lower than 2% by mole show excellent fracture toughness properties.
Consequently, the tendency described above indicates that a zirconia-based sinter having high fracture toughness properties can be obtained if a raw material containing a stabilizer in an amount smaller than 2% by mole (hereinafter abbreviated simply as "low-mole") is used and can be sintered at a low temperature.
However, it is extremely difficult to sinter zirconia with a low-mole stabilizer. In addition, zirconia-based sinters containing a rare earth metal oxide as a stabilizer generally have a drawback that they are susceptible to deterioration during long-term aging in a low-temperature region (the deterioration being caused by the transition of tetragonal crystals, which constitute a metastable phase at ordinary temperature among the crystal phases of the zirconia-based sinter, to monoclinic crystals, which constitute a stable phase, and by the development of minute cracks within the sinter as a result of a volume expansion accompanying the phase transition). In particular, aging in water or steam at 100.degree. to 300.degree. C. is a cause of considerable deterioration, and zirconia-based sinters stabilized with a low-mole stabilizer are more apt to undergo such a phenomenon.
The prior art techniques have therefore had a problem that a zirconia-based sinter having high toughness properties is difficult to produce. Even if a zirconia-based sinter having high toughness is produced, this kind of highly tough zirconia-based sinters have drawbacks of poor thermal stability in a low-temperature region and impaired product reliability and, hence, have a problem that the applications thereof are considerably limited.
Grinding part materials comprising a zirconia-based sinter excellent in strength and wear resistance have been proposed so far (see, e.g., examined Japanese patent publication No. 20587/1990). However, since these prior art zirconia-based sinters undergo a considerable decrease in strength upon long-term standing at around 100.degree. to 300.degree. C. and the rate thereof (rate of strength decrease) is exceedingly high especially in a water or steam atmosphere, the sinters raise difficulties, for example, in a wet grinding step using water as solvent or in the case where grinding part materials are washed with water etc., before being subjected to a drying step at a high temperature (around 200.degree. C.). As a sinter which eliminates such a problem, a zirconia-based sinter containing both a boric acid compound (e.g., B.sub.2 O.sub.3) and Al.sub.2 O.sub.3 and/or SiO.sub.2 [and further containing a rare earth metal oxide in an amount of 2% by mole or larger] has been proposed in unexamined published Japanese patent application No. 239662/1994 and others.
Such zirconia-based sinters show improved thermal stability. However, since these zirconia-based sinters contain 2% by mole or more rare earth metal oxide as a stabilizer, they neither show excellent fracture toughness properties comparable to that of zirconia-based sinters having a stabilizer content lower than 2% by mole, nor satisfy all the properties including fracture toughness properties and wear resistance.
On the other hand, bracket materials for dentition correction which comprise a zirconia-based sinter as a material similar to teeth in appearance and color tone and excellent in strength and toughness have been proposed in French patent No. 2,559,059, unexamined published Japanese patent application No. 21857/1990, unexamined published Japanese patent application No. 280864/1992, and others. For example, in unexamined published Japanese patent application No. 21857/1990 is described use of a zirconia-based sinter as a bracket material for dentition correction which sinter is a "so-called partially stabilized zirconia" partially stabilized with Y.sub.2 O.sub.3 or the like. In unexamined published Japanese patent application No. 280864/1992 is described use of a partially stabilized zirconia as a bracket material for dentition correction which zirconia contains a colorant comprising erbium oxide, praseodymium oxide, and iron oxide and is very similar in color tone to human teeth.
However, such prior art zirconia-based sinters also undergo a considerable decrease in strength upon long-term standing at around 100.degree. to 300.degree. C., and the rate thereof (rate of strength decrease) is exceedingly high especially in a water or steam atmosphere. The decrease of strength proceeds even at lower temperatures.
Medical materials including bracket materials for dentition correction raise difficulties, because these materials are especially frequently subjected at a high-temperature (around 100.degree. to 300.degree. C.) to cleaning with water as a solvent, disinfection, sterilization, or another treatment. As a sinter which eliminates such a problem, a zirconia-based sinter containing both a boron compound (e.g., B.sub.2 O.sub.3) and Al.sub.2 O.sub.3 and/or SiO.sub.2 [and further containing a rare earth metal oxide in an amount of 2% by mole or larger] has been proposed in Japanese patent application No. 169453/1994 and others. Such zirconia-based sinters show improved thermal stability. However, since these zirconia-based sinters contain 2% by mole or larger rare earth metal oxide as a stabilizer, they neither show excellent fracture toughness properties comparable to that of zirconia-based sinters having a stabilizer content lower than 2% by mole, nor satisfy all the properties including fracture toughness properties.
The present invention has been achieved in view of the drawbacks and problems described above. Objects of the present invention are as follows:
the first object is to provide a zirconia-based sinter which contains a low-mole rare earth metal oxide as a stabilizer, can be produced through sintering at a relatively low temperature, and is excellent in thermal stability and fracture toughness properties, and to provide a process for producing the same;
the second object is to provide a grinding part material which employs the zirconia-based sinter described above and a zirconia-based sinter obtained by the process for producing the same; and
the third object is to provide a bracket material for dentition correction which likewise employs the zirconia-based sinter described above and a zirconia-based sinter obtained by the process for producing the same.